Can there be a rainbow at night? The answer is yes, though this atmospheric phenomenon is exceptionally rare. This nighttime spectacle is known as a lunar rainbow or, more commonly, a moonbow. Like their daytime counterparts, these ethereal arcs are formed by the interplay of light and water droplets, but the specific conditions required make the moonbow a prize for dedicated sky-watchers.
How Sunlight Creates a Standard Rainbow
The formation of a rainbow begins with the sun positioned behind the observer, shining toward rain or mist. Each tiny water droplet acts as a miniature prism, initiating a three-step optical process that separates white sunlight into its constituent colors. The light first undergoes refraction, which is the bending of the light ray as it enters the denser water droplet.
This change in speed and direction causes the light to disperse, splitting white light into the visible spectrum because each color bends at a slightly different angle. Once inside the droplet, the light reflects off the back inner surface, a process called internal reflection. The light then refracts a second time as it exits the droplet and travels toward the observer.
The full primary rainbow arc is visible only at an angle of approximately 42 degrees from the point opposite the sun, known as the antisolar point. This precise geometry means that the sun must be low in the sky, less than 42 degrees above the horizon, for the light to emerge from the droplets at the correct angle to reach an observer on the ground. This combination of refraction, internal reflection, and dispersion is required for any rainbow, whether the light source is the sun or the moon.
Introducing the Lunar Rainbow
A lunar rainbow, or moonbow, is created by the exact same optical mechanism as a solar rainbow, but it uses the moon’s light as its source. Moonlight is not generated light; it is sunlight reflected off the lunar surface, making it significantly dimmer than direct sunlight. Because the moon reflects only a small fraction of the light it receives, the resulting moonbow is far less intense than a daytime arc.
The low intensity of the light has a profound effect on how the human eye perceives the moonbow. Under conditions of darkness, human vision shifts from using cone cells, which detect color, to using rod cells, which are better suited for low light but perceive only shades of gray. This reliance on rod cells is known as scotopic vision.
Consequently, moonbows often appear to the naked eye as pale white or monochrome arcs. While the full spectrum of colors is present, the light is too faint to stimulate the cone cells sufficiently to distinguish the individual hues. However, long-exposure photography, which allows a camera sensor to collect light over time, can reveal the vibrant colors hidden within the moonbow.
The Specific Conditions Needed for a Moonbow
The extreme rarity of a moonbow stems from the need for a strict alignment of both astronomical and meteorological factors. The first astronomical requirement is a very bright moon, meaning it must be at or very near its full phase to provide sufficient illumination. The moon’s light is approximately 400,000 times fainter than the sun’s, so any reduction in brightness makes the phenomenon impossible to see.
The Moon must also be positioned low in the sky, ideally less than 42 degrees above the horizon, mimicking the angle required for a solar rainbow. If the Moon is higher, the light emerges at an angle that does not reach an observer on the ground. Furthermore, a moonbow requires a very dark night sky with minimal light pollution, as artificial light sources can easily overwhelm the faint lunar light.
Meteorologically, there must be a source of water droplets in the air opposite the Moon, such as rain, mist, or spray from a waterfall. This combination of a nearly full moon, a low angle, a dark sky, and moisture makes the occurrence of a moonbow exceptional. The phenomenon is usually observed only in specific locations known for consistent mist, such as near major waterfalls.